Process for removing naphthenic acids from hydrocarbon oils



Patented Jan. 7, 1941 rnoonss FOR REMOVING NAPHTHENIG ACIDS FROM HYDROGARBON OILS Franz Rudolf Moser, Amsterdam, Netherlands, assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware No Drawing. Application May 13, 1939, Serial No. 273,575. In Great Britain May 23, 1938 1 Claim.

This invention is a continuation in part of my copending application, Serial No. 181,702, filed December 24, 1937, now Patent 2,186,425 and relates to the removal of naphthenic acids from 5 hydrocarbon oils by catalytic destruction of the latter at elevated temperatures, and in particular deals with the removal of naphthenic acids from mineral lubricating oil distillates.

Hydrocarbon oils often contain considerable amounts of acidic components, which during the processing of the oils, particularly at elevated temperatures, may cause severe corrosion of the treating equipment. Thus corrosion difliculties have been experienced, for instance, when distilling, with or without steam, relatively high boiling oils such as gas oil, Diesel fuel oils, lubricating oils, etc., containing free naphthenic acids, particularly if their acid numbers are above about .2. In order to overcome this difiiculty various means have been employed, all of which, however, have certain disadvantages. For instance it has been attempted to subject the oils to selective cracking at temperatures between about 315 C. to 400 C. whereby it was hoped the naphthenic acids would be decomposed to neutral compounds and C02, while the hydrocarbon oils remain substantially uncracked. It was found that under these conditions some decomposition of naphthenic acids takes place, but it is slow and incomplete. Moreover the corrosion during this treatment may be severe and may persist in subsequent treatments.

Another method consisted of passing vapors of mineral oil containing naphthenic acids over alkaline earth metal oxide at temperatures of about 200 C. to produce alkaline earth naphthenates, and heating the naphthenates so produced to about 400 C. to remove therefrom ketones and CO2, thereby restoring the alkali earth oxide. This intermittent process has the disadvantage that it requires frequent changes of temperatures of the reacting masses, which reduce the available operating time, make operating of the process on a large scale difiicult, and raise operating expenses.

Other methods of removing naphthenic acids comprise distilling oils containing naphthenic acids, for instance, over caustic soda under conditions to retain the naphthenic acids in the alkaline-reacting residue in the form of a socalled soda asphalt, which, however, is a troublesome material of little use.

It is a purpose of this invention to provide a continuous method whereby naphthenic acids contained in mineral oil can be eliminated quickly and substantially completely, without produc ing an undesirable residue and without materially decomposing or otherwise damaging the hydrocarbons with which the acids are associated.

I have discovered that naphthenic acids can be eliminated quickly and substantially completely in a single step operation from mineral oils containing them by conducting the oil, preferably in the vapor phase, at an elevated temperature, if desired in the presence of steam and under conditions substantially to avoid cracking of the hydrocarbons over a mixed catalyst mass containing both inorganic alkali metal compounds and alkaline earth compounds, whereby the harmful carboxyl groups of the naphthenic acids are catalytically destroyed or converted into nonacidic groups.

The reaction by which the naphthenic acids are eliminated is one of decomposition, free CO2 and neutral compounds, presumably ketones and/or hydrocarbons being formed.

The metal compounds in the catalyst are preferably in the form of their oxides or carbonates, or of compounds capable of forming such oxides or carbonates under the reaction conditions. To be suitable the compounds should be solid at the reaction temperatures. Thus an active catalyst may comprise an oxide or carbonate of lithium, sodium and/or potassium together with an oxide or carbonate of magnesium, calcium, strontium and/or barium. Such a catalyst contains side by side, substances having a strongly alkaline reaction, 1. e. the alkali metal compounds, and substances having a less strongly alkaline reaction, i. e. the alkaline earth metal compounds.

In accordance with my invention I have found such mixtures to have a better catalytic effect than each of the substances individually. A possible explanation may be as follows: The action of the catalyst is conceivably split up into two successive stages, e. g. (1) The binding of the naphthenic acids and, (2) The decomposition of the bound naphthenic acids. For binding the naphthenic acids the alkali metal compounds, being the more strongly basic substances, are most desirable. However, the naphthenates of the alkaline earths decompose at lower temperatures and melt at higher temperatures then the naphthenates of the alkali metals, and hence the presence of the alkaline earth compounds materially accelerates the decomposition of naphthenic acids and simultaneously stabilizes the catalyst. Consequently to aid in the decomposition of the naphthenates and to give the catalyst a higher melting point to prolong the activity of its surface, alkaline earth metals are desirable. It is, therefore, of advantage to use mixtures of the said substances as catalysts.

In order that both reaction stages may proceed properly, it is desirable that both the alkali metal compound and the alkali earth compounds be present in substantial proportions. Thus it is desirable that the ratio of the two types of compounds be between not less than 1:20 not more than 20:1 and preferably be between the limits of 1:10 and 10:1. An apparent exception is found in the case of lithium, which in concentrations of 1% and lower is capable of activating alkali earth compounds as disclosed in the copending application, Serial No. 181,702.

The catalysts are preferably prepared in such a manner as to ensure a large active surface and high porosity. This object may be obtained by grinding the oxides and/or carbonates to a very fine particle size. Or else suitable carbonates of the alkali metals and alkaline earth metals may be obtained from solutions of soluble salts by precipitation if necessary in the presence of a precipitant such as lower alcohols, or by calcining or igniting their bicarbonates, or carboxylic acid salts which upon heating will decompose to form carbonates, in particular acetates and higher fatty acid salts and naphthenate's. Carbonates obtained by heat treatment are usually more efficient than the precipitated forms. One may also start from a compound yielding the desired catalyst after decomposition. Thus a very active catalyst is produced by heating the double compound of magnesium carbonate and potassium carbonate (the Engel salt) at about 400 C.

The active catalysts may be deposited on carriers such as coal, coke, concrete, pumice, metals, (for example copper gauze and steel wool) etc. and if desired may be mixed with activators. Thus the addition of small amounts of certain substances, for example, oxides of copper, silver, zinc, cadmium and manganese, may serve as activators.

Suitable contact temperatures range from about 300 C. to temperatures of incipient cracking or slightly above, i. e. about 475 C. Incipient cracking temperatures and rates of cracking at a given cracking temperature, vary somewhat with the boiling range and the nature of the oil as shown by Geniesse and Reuter in Industrial and Engineering Chemistry 24 (1932) pages 219 etc. Exposure of the oil to temperatures above incipient cracking temperatures should be so limited to avoid substantial cracking. In general treating temperatures should not exceed about 475 C. and are preferably below about 450 C.

The limited time of contact available when treating without substantial cracking at temperatures above incipient cracking, is, however, more than sufficient to eliminate the largest portion of naphthenic acids contained in hydrocarbon oils. For instance, whereas at 450 C. it requires between 3 to 4 seconds to crack a gas oil to the extent of 1%, a contact time of ,4 second is usually sufficient at temperatures from 400 C. to 450 C. to remove the naphthenic acids practically quantitatively with an active mixed alkali and alkaline earth carbonate catalyst. Since in some cases a longer time of contact may result in a more complete removal of the naph thenic acids, I may extend this time to several seconds, where this can be done without the danger of cracking or otherwise adversely affecting the oil. The following examples further illustrate my process.

A catalyst suitable for the process of my invention was prepared by thoroughly mixing 80 parts by weight of anhydrous soda and 20 parts by weight of voluminous magnesium oxide in a ball mill, compressing the mixture to pills and subsequently breaking the said pills and reducing them to a grain size A. S. T. M. sieve /20. A lubricating oil distillate containing naphthenic acids and having an acid number of 2 mg. KO'H per gram of oil, produced by distillation of a Venezuelan crude oil, was passed in the vapor phase at a pressure of 100 millimeters Hg. and temperature of 400 C. and in the presence of 2.5% by weight of steam over 10 cm. of this catalyst at a rate of 100 cm. per hour. The calculated time of contact was second.

The treated oil vapors were condensed and small amounts of gaseous CO2 were separated from the liquid. The naphthenic acids were found to be removed practically quantitatively even after 100 hours of catalyst life.

When substituting a sodium carbonate catalyst free from magnesium oxide in the above experiment which had been prepared in a similar manner, the de-acidifying effect at first was the same as that of the catalyst described above, but after only a few hours the activity decreased considerably.

When magnesium oxide free from sodium carbonate was used as a catalyst in an experiment similar to the above, the de-acidification was at the outset markedly less intensive that in the case of a sodium carbonate catalyst or a catalyst composed of sodium carbonate and magnesium oxide. In addition to being less effective than the above catalysts, the activity of the magnesium oxide catalyst decreased very rapidly.

I I claim as my invention:

In the process of removing naphthenic acids from hydrocarbon oils containing same, the steps comprising contacting said oil in a heated vaporous condition at a temperature between 300 C. and 475 C. with a catalyst comprising calcium carbonate containing about 1% lithium carbonate, fora time suificient to decompose a major portion of the naphthenic acids, thereby liberating gaseous CO2, but insufiicient to substantially crack the oil.

FRANK RUDOLF MOSER. 

